Catalytic cracking processes become increasingly important. In a catalytic cracking process frequently a cyclic operation of the cracking catalyst is used. In this cyclic operation, the catalyst is advanced through a loop from a cracking zone through a separation zone and a regeneration zone back to the cracking zone. In the cracking zone, the cracking catalyst is contacted with the hydrocarbon feedstock under cracking conditions to produce lower molecular weight hydrocarbons. In the separation zone, the cracking catalyst and the hydrocarbons are separated. In the regeneration zone, the cracking catalyst is contacted with a free oxygen containing gas to burn off any coke present on the cracking catalyst and to restore the cracking activity of this catalyst.
During the process, the cracking catalyst particles generate cracking catalyst fines which are difficult to separate from the gases. The usual operation applied in the industry involves a separation of the fines from the hydrocarbon product and these fines remain either in the separator or are recombined with the main larger cracking catalyst particles in the cracking catalyst loop described above. The fines also, to a certain extent, are separated in the regenerator. Since fines are constantly generated in the process, it is necessary to either employ a very efficient method for separating the fines or to let a portion of the fines escape through the regenerator stack or with the hydrocarbon product. In today's commercial cracking operations, usually both paths of loss of cracking catalyst fines do occur. In a typical cracking operation using a catalytic cracker containing 300 tons of circulating cracking catalyst, it is not uncommon that daily, between 6 and 10 tons of catalyst fines leave the loop in part through the regenerator stack and in part with the hydrocarbon product.
It is also well known in the art to subject the hydrocarbon product from the cracking zone and containing some of the cracking catalyst fines to an initial fractionation step. Most of the cracking catalyst fines contained in the cracked hydrocarbon product will leave this fractionation step with the bottoms product. This bottoms product is known to be useful as a carbon black feedstock. However, the quality of the carbon black made with such a feedstock depends to a significant extent upon the content of catalyst fines. These catalyst fines form ceramic grit which is undesirable for many of the modern applications of the carbon black and customer specifications for the carbon black therefore frequently set an upper limit for such grit content. In some instances, it has therefore become necessary to subject the bottoms stream from the first fractionator to a separation step to separate as much of the cracking catalyst fines from the oil as possible prior to its use in a carbon black production operation. Such a separation step is, of course, costly and therefore undesirable.